The present invention relates to optical devices having electrically addressable composite layered flakes with angularly dependent optical properties in a system including the flakes and a host fluid. The composite flakes have a plurality of layers of different materials, such as dielectric, conductive and polymer liquid crystalline materials. The optical properties are electrically selectable or addressable by an electric field applied toward the flake/host system.
Reference may be made to the following patent documents which describe optical devices containing flake/host systems that rely on polymer liquid crystal (PLC) flakes to provide angularly dependent optical effects: U.S. patent application Ser. No. 09/571,805, filed May 16, 2002 and published in International Publication WO 01/88607, published Nov. 22, 2001, now U.S. Pat. No. 6,665,042, issued Dec. 16, 2003; and U.S. patent application Ser. No. 10/405,163, filed Apr. 2, 2003 by Tanya Z. Kosc, Kenneth L. Marshall, and Stephen D. Jacobs. Both of these patent documents are incorporated herein by reference.
Particles of various materials have been used, principally in devices relying on electrophoretic effects, in order to provide electrically switchable optical devices. These particles are generally all dielectric and do not rely on composite layers to obtain optical effects or to enhance particle motion as is the case with devices of the present invention. Reference may be made to the following U.S. Patents for further information regarding such prior devices: Labes, U.S. Pat. No. 4,657,349, issued Apr. 14, 1987; Albert, U.S. Pat. No. 6,392,785, issued May 21, 2002; Jacobson, U.S. Pat. No. 6,422,687, issued Jul. 23, 2002; Sheridon, U.S. Pat. No. 6,497,942, issued Dec. 24, 2002; and Albert, U.S. Pat. No. 6,515,649 issued Feb. 4, 2003. Other devices similar to those in the referenced patents, as well as other documents that describe them, are referenced in the above-identified International Publication.
It has been discovered in accordance with the invention that composite or layered flakes that have heretofore been used exclusively as paint and ink pigments and have angularly dependent optical properties, can be selected or addressed by an electric field when contained in a flake/host system.
The composite flakes used in a flake/host system in accordance with the invention may include both dielectric and conductive layers. The dielectric layers may include, but are not limited to, materials such as mica, titanium dioxide, rutile, aluminum oxide, silica, magnesium fluoride, polymers, Mylar (polytetrafluorethylene), cellophane, polyester, and polyethylene and polymer liquid crystals (PLC""s). Metals such as gold, silver, aluminum, tin, and metal oxides such as tin oxide or indium tin oxide may be used as the conductive layer, but the conductive layer is not limited to these materials.
The angularly dependent optical properties may be obtained in several ways: (1) one or more of the layers can be liquid crystalline; (2) two or more layers can reflect light, causing optical interference effects as determined by the index of refraction and the thickness of the layers; (3) one or more layers can contain holograms; (4) any combination of these methods can be used.
Though angularly dependent optical effects are often desired, there are conditions in which it is preferable to suppress or alter the angular dependence. This suppression of angular dependence can be achieved by preparing composite flakes with a specifically colored layer, such as a dyed polymer, coated with a layer having angularly dependent optical properties, such as a liquid crystal, and utilizing additive and/or subtractive color effects. The composite flake reflects a specific color when it is illuminated and viewed normally. However when the composite flake is viewed off-axis, the constant color reflecting from the bottom layer compensates for the shifting color being reflected from the top liquid crystalline layer, minimizing the change/shift in apparent color. A specifically colored bottom layer in a composite flake is not limited to suppression of angle-dependent color effects. Specific color combinations can also alter, intensify, and enhance color dependent effects.
A composite flake may also serve to manipulate the effects of circular polarization that are obtained when polymer cholesteric liquid crystal (PCLC) layers are used. PCLC""s have an inherent optical property known as selective reflection that causes a PCLC layer to reflect only light of a specific wavelength (color) and circular polarization (right- or left-handed). The ability to provide polarized light is highly desirable for many applications, particularly those employing three-dimensional effects or tagging for security features. Because selective reflection only reflects one handedness of light, the opposite handedness, or 50% of the light, is not utilized. In applications where brightness of the reflecting layer is a primary concern, it would be useful to manufacture composite flakes possessing two layers of PCLC, with each layer reflecting one of the two polarizations of light, thus maximizing flake reflectivity.
The use of a metallic layer may be two fold. In certain inks and pigments, the metallic layer is required to produce the desired optical effect, particularly for xe2x80x9cpearl lusterxe2x80x9d or xe2x80x9cpearlescentxe2x80x9d pigments. When a metallic layer is included in the slack (composition) of layers in a composite flake, it will change the dielectric properties of the flakes. Since the metallic layer is conductive, it significantly alters flake behavior when an electric field is applied.
The plurality of layers in composite flakes can be dielectric and can be composed of two or more layers. For example, dielectric layers can be used to xe2x80x9csandwichxe2x80x9d a metal oxide layer or a metal layer. One or more of those dielectric layers may be a PLC (either a PCLC, a polymer nematic liquid crystal, or a polymer smectic liquid crystal). The flakes may be produced by the methods described in the examples that appear below.
Various commercial pigments may be used to provide the composite flakes. Some of such pigments and their manufacturers are listed in the following table. In the table, xe2x80x9cpearl lusterxe2x80x9d thin film pigments are the traditional pigments based on mica platelets coated with aluminium or bronze layers whose thickness determines the color of the interference effect. xe2x80x9cMultiple layerxe2x80x9d thin film pigments refer to pigments that use multiple layers (typically dielectric) for interference effects.
In addition, the following United States Patents describe composites of layered flakes for pigments used in paint and ink: Venis, U.S. Pat. No. 4,168,986, issued Sep. 25, 1979; Phillips et al., U.S. Pat. No. 5,279,657, issued Jan. 18, 1994, Phillips et al., U.S. Pat. No. 5,571,624, Issued Nov. 5, 1996; Hou et al., U.S. Pat. No. 5,587,242, issued Dec. 24, 1996; Miekka et al., U.S. Pat. No. 5,672,410, issued Sep. 30, 1997; Phillips et al., U.S. Pat. No. 5,766,738, issued Jun. 16, 1998. Pigments using PCLC and other layers appear in Muller-Rees et al., U.S. Pat. No. 5,851,604, issued Dec. 22, 1998; Poetsch et al., U.S. Pat. No. 6,291,065, issued Sep. 18, 2001; U.S. Patent Application Publication, Inoue et al., U.S. 2002/0033117, published, Mar. 21, 2002. The foregoing patents also describe methods of making the composite layered flakes.